Multi-speed power transmissions, particularly those using planetary gear arrangements, require a hydraulic system to provide controlled engagement and disengagement, on a desired schedule, of the clutches and brakes or torque transmitting mechanisms that operate to establish the ratios within the planetary gear arrangement.
These control systems have evolved from substantially pure hydraulic control systems, wherein hydraulic devices, produce all of the control signals to electro-hydraulic control systems, wherein an electronic control unit produces a number of the control signals. The electronic control unit emits electrical control signals to solenoid valves, which then issue controlled hydraulic signals to the various operating valves within the transmission control.
With many of the early pure hydraulic and first generation electro-hydraulic control systems, the power transmission utilized a number of freewheel or one-way devices which smooth the shifting or ratio interchange of the transmission during both upshifting and downshifting of the transmission. This relieves the hydraulic control system from providing for the control of overlap between the torque transmitting mechanism that was coming on and the torque transmitting mechanism that was going off. If this overlap is excessive, the driver feels a shudder in the drivetrain, and if the overlap is too little, the driver experiences engine flare or a sense of coasting. The freewheel device prevents this feeling by quickly engaging when the torque imposed thereon is reversed from a freewheeling state to a transmitting state.
The advent of electro-hydraulic devices gave rise to what is known as clutch-to-clutch shift arrangements to reduce the complexity of the transmission and the control. These electro-hydraulic control mechanisms are generally perceived to reduce cost and reduce the space required for the control mechanism.
In addition, with the advent of more sophisticated control mechanisms, the power transmissions have advanced from two-speed or three-speed transmissions to five-speed and six-speed transmissions. The torque capacity of a torque transmitting mechanism (on-coming or off-going) involved in a shift may be conveniently controlled by the combination of an electrically activated solenoid valve and a pressure regulator valve or trim valve. In a typical system, the solenoid valve is activated by pulse-width-modulation (PWM) at a controlled duty cycle to develop a pilot pressure for the pressure regulator valve or trim valve, which in turn, supplies fluid pressure to the torque transmitting mechanisms in proportion to the solenoid duty cycle.
Additionally, an electrically variable hybrid transmission has been proposed to improve fuel economy and reduce exhaust emissions. The electrically variable hybrid transmission splits mechanical power between an input shaft and an output shaft into a mechanical power path and an electrical power path by means of differential gearing. The mechanical power path may include clutches and additional gears. The electrical power path may employ two electrical power units, or motor/generator assemblies, each of which may operate as a motor or a generator. With an electrical storage system, such as a battery, the electrically variable hybrid transmission can be incorporated into a propulsion system for a hybrid electric vehicle.
The hybrid propulsion system uses an electrical power source as well as an engine power source. The electrical power source is connected with the motor/generator units through an electronic control unit, which distributes the electrical power as required. The electronic control unit also has connections with the engine and vehicle to determine the operating characteristics, or operating demand, so that the motor/generator assemblies are operated properly as either a motor or a generator. When operating as a generator, the motor/generator assembly accepts power from either the vehicle or the engine and stores power in the battery, or provides that power to operate another electrical device or another motor/generator assembly.
There are two main hybrid vehicle architectures—the parallel hybrid and the series hybrid. The hybrid electric vehicle with a parallel configuration has a direct mechanical connection between the hybrid propulsion system and the drive wheels of the vehicle. In contrast, the hybrid electric vehicle with a series configuration uses an engine mounted generator to produce electricity for the batteries and/or the electric motor. The series hybrid electric vehicle has no mechanical connection between the hybrid propulsion system and the drive wheels. Some hybrid propulsion systems may selectively operate in either a parallel or a series configuration by employing a clutching mechanism, such as a dog clutch.
Additionally, modern control systems allow “shift by wire” range shift capability. Another name for this technology is electronic transmission range selection, or ETRS. The ETRS systems dispense with much of the mechanical interconnections found in mechanically actuated transmissions, thereby simplifying the transmission architecture and internal mechanical shifter designs. The additional functionality provided by an electrically variable hybrid transmission requires creative methods of control architecture to reduce cost, complexity, and weight, while increasing reliability.